1. STOP for a moment. Observe and Name the experience you are having "I am freaking out". This already starts to turn on your prefrontal cortex.

2. Grab ice cubes and hold them as long as you can. YIkes! You won't last long. (An alternative would be to pop a warhead candy or a lemon/lime slice into your mouth).  This will trick your brain in a helpful way.

3.  Place the entire palm of your hand over your forehead, gently pressing for 1 - 5 minutes. Until you feel better. This sends signals for blood to comes back to the prefrontal cortex. The trick is it can take several minutes. 

4. Take 3 full belly breaths. As best as you can. You have now tolerated the worst of it, using your skills, and can regulate the emotional experience. You may reflect to gain insight. Write about it. Start problem solving. Or distract yourself. 
Working with the urge to lash out.